In general, an injection molding machine is provided with an ejector mechanism for taking out a molded article from a mold. As shown in FIG. 3, by way of example, an ejector mechanism installed in a motor-driven injection molding machine is arranged to drive an ejector pin 10', which extends through a movable platen 1', by means of a drive mechanism 20' including a motor 24', so as to push out a molded article to the outside of a mold. A ball screw 21' of the drive mechanism 20' is screwed into the ejector pin 10', and a ball nut 22' threadedly engaged with the ball screw 21' is rotatably supported by a casing 30' fixed to the movable platen 1' and is coupled to the motor output shaft through a belt 23'. The ejector mechanism further includes a plurality of guide rods 40' extending in parallel with the ejector pin 10', and a guide plate 41' arranged for reciprocal motion between the movable platen 1' and the casing 30' along the rods 40'. The ball screw 21' is securely fixed at a front end thereof to a central portion of the plate 41' by a key 42', so as to be unrotatable. As the motor 24' rotates, the ball nut 22' is rotated, so that the ejector pin 10' is moved toward the mold in unison with the ball screw 21', to thereby push out the molded article.
In the above-mentioned ejector mechanism, it is essentially required to provide the plurality of guide rods 40' and the guide plate 41', in order to accurately move the ejector pin 10' along the ejector axis 100'. This, however, leads to an increased number of parts of the ejector mechanism and to complicated structure. Moreover, these constituent parts of the ejector mechanism must be manufactured and the ejector mechanism must be assembled with such a high precision that the ejector pin 10' and the guide rods 40' are exactly parallel with the axis 100' and at the same time an error in the distance between the axes of adjacent guide rods 40' falls within a stringent allowable range of .+-.20-30.mu..